Daylight harvesting refers to the use of natural daylight to supplement the artificial light in an environment such as an interior space of a building, e.g. an office or other room. The idea can be used to reduce the amount of artificial light needed to illuminate the space and so reduce energy consumption. Nonetheless, in certain environments such as an office workspace a certain standardised or recommend light level may be required, e.g. 500 Lux at desk height. Even in non-regulated environments the end-user may require a certain light level as matter of preference. Hence to conserve energy consumption whilst still meeting the relevant light level requirement, a modern lighting system may comprise a controller which adjusts the artificial light output by one or more electric lighting devices depending on the amount of daylight present.
A closed-loop control system uses feedback of the quantity it is controlling (as opposed to an open-loop system which does not use feedback). In the case of a lighting control system, a photosensor detects the total photometric amount of light from both daylight and electric sources in the space. The sensed level is then used to control the amount of light. Some systems may use “active daylighting” whereby a suitable device is controlled to admit or direct varying amounts of daylight into the space in question, e.g. by means of automatic window blinds, windows comprising “smart glass” with controllable opacity, or a heliostat comprising a movable reflector arranged to direct a controllable amount of light through a window or other opening. In addition artificial light from one or more electric lights may be adjusted to make up any requirement for overall light level that cannot be met using the natural daylight alone. Other systems may passively collect natural light though regular windows or skylights and adjust the electric light sources to meet the required light level.
U.S. Pat. No. 8,207,676 discloses an arrangement in which two different light sensors are integrated within an LED-based light source. The first sensor is provided with a diffuser arranged to collect both light from the LED light source and ambient light, whereas the second sensor is provided with sidewalls that block light from the LED source so as to only collect ambient light. These inputs are used to regulate the light output of the LED light source.